The crumbling neighborhood public school down the block or that gilded private school on a hill? There's a tendency to imagine the two this way - and to assume the private school will produce better students.

But beleaguered public schools have recently received a small, though noteworthy, boost. After accounting for students' socioeconomic background, a new study shows public school children outperforming their private school peers on a federal math exam.

Overall, private school students tend to do markedly better on standardized tests. But the reason, this study suggests, may be that they draw students from wealthier and more educated families, rather than because they're better at bolstering student achievement.

One study is unlikely to settle a long-simmering debate over the merits of public versus private education. But its authors say they hope it will give pause to a current trend in education reform: privatization.

From tax-dollar financed vouchers for private schools to a drive to put public schools in private hands, market-style reforms are all the buzz in education.

Competition, the reasoning goes, is healthy for schools. Those that must produce results to survive have to be better than those that don't face such pressure.

But these findings "really call into question the assumption of some of the more prominent reform efforts," says Christopher Lubienski, an education professor at the University of Illinois at Urbana-Champaign, who wrote the study with his wife Sarah Theule Lubienski, also an education professor at the university.

In particular, says Mr. Lubienski, it challenges the assumption that "the private-school model is better and more effective, and can achieve superior results. It really undercuts a lot of those choice-based reforms."

"A New Look at Public and Private Schools: Student Background and Mathematics Achievement" appears in the May issue of Phi Delta Kappan, a highly regarded education journal.

Analyzing raw data from the 2000 National Assessment of Educational Progress for 28,000 fourth- and eighth-graders representing more than 1,300 public and private schools, Mrs. Lubienski, whose research focuses on equity issues in math education, was surprised by what she was seeing. When children of similar socioeconomic status were compared, the public school children scored higher. She called in her husband, who studies school choice and privatization, to help interpret the results.

At most, a small difference

When the students were divided into four socioeconomic groups, the difference between public and private school math scores was 6 to 7 points for fourth-graders in each group, and 1 to 9 points for eighth-graders. Not a large difference, says Mrs. Lubienski - more "small to moderate."

The crux of the study isn't so much to suggest that public schools are outpacing private schools as to call into question a common assumption: "It's more: 'Wow, this flies in the face of what we thought - that private schools do better than public.' "

Some, however, are skeptical. Any time studies produce "counterintuitive" results, they should be carefully examined, says Joe McTighe, executive director of the Council for American Private Education (CAPE).

Without having seen the Lubienskis' research, he points out that raw scores have typically shown the country's 6 million private school students, who make up 11.5 percent of US schoolchildren, outperforming public school students.

But others say the results are not that surprising.

"We would conclude, on the basis of perhaps 15 years of research, that there's nothing magic about privatization," says Henry Levin, a professor of economics and education at Columbia University's Teachers College in New York, and director of the National Center for the Study of Privatization in Education.

The 'peer effect'

For the most part, he says, the academic benefits of attending a public or a private school have been relatively small, when compared in studies.

Certainly the stubborn gap between the academic achievement of white and minority students across the board is greater than the gap between public and private school performances.

That's a salient point for those who oppose the idea of privatization as a cure for the ills of US public education. Unless private schools have a measurably better track record than public schools, it's hard to argue that privatizing education will necessarily boost performance for all students.

"The bigger picture here," says Professor Levin, "and I don't care which good study you look at - the [differences in public and private school] results are tiny."

By way of example, Levin cites a comprehensive 2002 study that examined public and private schools in Latin America.

There, raw test scores favored private schools. But once student socioeconomic status was taken into account, that advantage shrank - as the Lubienskis had found. After the study factored in what is called "peer effect" - the influence of other students and school environment - the overall difference across 10 countries was zero; achievement at public and private schools was equal.

In the end, says Levin, ideology often trumps research and drives the debate, with proponents on either side highlighting only data that support their case.

Parents opt for intangibles

Chances are, the Lubienskis' study won't have much impact on the choices that parents make.

"When parents make decisions about schools," says Mr. McTighe of CAPE, "they don't compare these constructed statistical abstracts. They look at a particular school in a particular neighborhood and ask: Is that school the right match for my child?"

Often, intangibles - a safe environment and caring staff or a culture that embraces and reflects a family's values - influence the decision as much as test scores.

And public opinion about the quality of schools is nuanced.

According to a 1999 Public Agenda poll, 52 percent of parents said private schools generally provide a better education. Only 19 percent thought a public education was better.

Yet many parents still hold public schools in high regard. The annual Phi Delta Kappa/ Gallup Poll of the Public's Attitude Toward the Public Schools has, since 1983, consistently reported parents giving public schools high marks - in the "A" and "B" range. The closer a parent is to the school, the better the grade.

If nothing else, the Lubienskis' findings are "a nice thing for the public schools to hear," says Kathy Christie of the nonpartisan Education Commission of the States in Denver. "It's most validating in that way."